More on Dr. Thompson's Thesis Work
David's Ph.D. thesis was on atmospheric gamma radiation. It has long been known that the Earth is bombarded by huge numbers of charged particle cosmic rays, mostly protons, electrons, and alpha particles. It was also known that these cosmic rays interact with the atoms in the Earth's atmosphere to produce even more particles, including pi mesons, mu mesons, and gamma rays. In fact, even at the highest altitudes that can be reached by scientific balloons (about 130,000 feet), the secondary gamma rays outnumber the cosmic gamma rays that can tell us about high-energy phenomena in astrophysical sources. When he started working at Goddard, the gamma-ray group was building the first imaging gamma-ray satellite telescope, SAS-2.
David's advisor, Carl Fichtel, realized that some of these many cosmic rays hitting the atmosphere would produce gamma rays that scattered upward or sideways and could potentially interfere with the satellite once it was launched. His thesis was a combined experimental and modeling effort to understand the atmospheric gamma radiation in three dimensions. David refurbished and flew a small gamma-ray telescope on a balloon, looking sideways and downward to measure the scattered gamma radiation. He also developed a Monte Carlo model of the showers of interactions the cosmic rays made in the atmosphere, tracing the particles in a 3-D way in order to compare with the observations.
He found that the earth's atmosphere is a bright source of gamma rays, and the horizon is particularly bright. This has been an important consideration in all succeeding gamma-ray telescopes, which have to avoid looking at the earth's horizon or at least not trying to do astronomy when the horizon interferes with the celestial observations. The Monte Carlo calculation also suggested that the Moon could be a source of gamma rays, through the same sort of cosmic ray collisions that take place in the atmosphere. It was not until EGRET flew that that prediction was confirmed.